Export flow types (Options, CacheMap, and BatchLoadFn)

[rwe]

The type definitions are helpful, but because they're not exported, it makes them difficult to use in composed APIs.

The only real change request here is to add export to those type definitions.

[erictt]

If you are using flowtype, it probably will warn you when your CacheMap Class doesn't work.

[rwe]

My suggestion here isn't about lack of type checking, only that you can't reference those unexported types in your own declarations. Like:

import DataLoader from 'dataloader';

export default class MyDataLoader extends DataLoader<string, MyType> {
  constructor(someArg: string, options: /* what type declaration to put here? */) {
    super(someBatchLoadFn, options);
  }

  someHelperMethod(): void {
  }
}

If they were exported, then one could just:

import type {Options as DataLoaderOptions} from 'dataloader';

[Togrias]

Also, Flow seems to type-check incorrectly if you simply define:
type BatchLoadFn<K,V> = $ReadOnlyArray<K> => Promise<$ReadOnlyArray<V | Error>>

Somehow, Flow can't resolve (V | Error) properly. So new DataLoader(fn: BatchLoadFn) shows a warning.

The problem goes away if you modify the latest git commit to export the flow types and depend on those exported flow types.

But that seems like a bug with Flow rather than this.